This invention relates generally to the manufacture of manifolds for multi-cavity injection molding systems, and more particular to an improved method of manufacturing a composite manifold having a tortuous melt passage with smooth bends and an integral heating element.
Manifolds of this general type have a common melt inlet opening on one external surface and a number of outlet openings on an opposite external surface. The melt passage in the manifold through which the melt from the inlet opening flows has several branches leading to the outlet openings. Furthermore, it is usually critical that the height or thickness of the manifold between the inlet and outlet openings be kept to a minimum. Therefore, the melt passage must necessarily be very tortuous and have a number of bends. However, in order to avoid unacceptable pressure drop and shear which can cause decomposition when molding certain engineering materials such as polyvinyl chloride and some polyesters, it is essential that the rheology of the melt passage configuration be adequate to provide streamlined flow by avoiding sharp bends or corners. Otherwise, excessive pressure drop and decomposition of the melt increases the cost of the system itself and reduces the quality of the product produced by the system.
In the past, manifolds having a branching melt passage configuration such as this have been made by first drilling cross bores in the manifold, inserting welded plugs as required to close them off, and then drilling perpendicular intersecting bores (as shown in FIG. 1). In addition to necessarily forming sharp bends where the bores intersect and being labour intensive and therefore costly, this method has the disadvantage that the welded plugs occasionally crack and leak which requires the whole system to be shut down.
While attempts have been made to overcome these problems by using split manifold blocks which are clamped together during use, they have had the problem that the very considerable molding pressure of the melt often results in leakage. One example of this structure is shown in U.S. Pat. No. 3,923,209 to Roy which issued Dec. 2, 1975 and discloses a manifold with pie-shaped sectors arranged in a hub to withstand the pressure. However, it will be appreciated that this manifold configuration is not practical for many applications.
It is, of course, well known to provide the manifold with an electrical heating element to maintain the melt at a uniform temperature as it flows therethrough. As shown in the applicant's Canadian Pat. No. 1,174,020 which issued Sept. 11, 1984, it is even known to form an integral manifold by casting a heating element into a channel in the surface of the manifold in a vacuum furnace. However, the improved method of the present invention has the advantage that the split manifold plates are sealed together against leakage and a heating element is cast into the manifold in a single vacuum heating step. In addition to forming a manifold with excellent flow and temperature characteristics, this method considerably reduces manufacturing time and cost.